Not at all. Today’s featured species of catfish is Phreatobius cisternarum, a Brazilian fish which is lives in phreatic habitats. Hydrologists will be sitting up in alarm because this means the catfish lives beneath the water table: it is literally an underground catfish. When people in parts of Brazil dig wells they find this catfish is already there! Although to be fair, the fishes are discovered in shallow wells but do not live deep down in confined aquifers (at least not that we know of).

The Water Table with Confined and Unconfined Aquifers (USGS)

Phreatobius cisternarum lives both to the north and the south of the Amazon River delta as well as on the Island of Marajó (a freshwater island approximately the same area as Switzerland). It is not a large fish. The biggest specimens grow to 2.2 inches in length (5.5 centimetres). Phreatobius cisternarum also does not seem to be a particularly gregarious catfish: individuals pass their time in the darkness hiding motionlessly in crevices waiting for the macro-invertebrates they feed on (worms being a particular favorite). With broad heads, vestigial eyes, and vermiform fins, there is something chthonic about these little catfish.

The fish are a distinctive blood red color because they exchange oxygen through their gas permeable skin. Not only do the tiny fish utilize cutaneous respiration, they also are known (like many catfish) to sip air for its gaseous oxygen. Living in subterranean waters which have little soluble oxygen, these catfish have learned to maximize every atom of the precious gas. For a long time Phreatobius cisternarum was the only known member of this genus, but in 2007 two new Phreatobius species were discovered underground. These two species, P. dracunculus and P. sanguijuela, are entirely eyeless. Little is known ofPhreatobius cisternarum: the creature’s mating rituals, lifespan, and habits remain a mystery. Even less is known of the two new species.